U.S. Pat. No. 5,633,715 describes a method for determining the height location of a number of spatial positions on a sample defining a height map of a surface through white light interferometry with a broadband light source. The method involves for each spatial position on the sample the steps of obtaining a interference pattern signal or correlogram during scanning of the surface in a direction perpendicular to said surface using white light interferometry and estimating the point of the interference pattern signal where the amplitude of the correlogram is at its maximum, thus determining an approximation of the height location of a specific spatial position on the sample represented by said correlogram.
In the apparatus described in U.S. Pat. No. 5,633,715, the estimation of the value where the correlogram reaches its maximum takes place by calculating the center of mass of the derivative of the correlogram. Although this may lead to good results, this method is very sensitive for errors and noise caused by the data gathering process during scanning. The method is a coherence method, rather than a phase method, for determining a height map. The use of the derivative of the correlogram leads to emphasis of high frequency components and hence to amplification of errors.
The coherence methods and apparatus described above provide only a rough approximation of the apex of the correlogram and hence of the height location of a specific spatial position on the sample. In many situations, the height needs to be determined more accurately.
U.S. Pat. No. 5,398,113 discloses the use of phase signal to find a height map. In the method, the phase in Fourier domain is used to determine the height. A zero crossing of the inverse Fourier signal, however, is not used.
U.S. Pat. No. 6,775,006 and U.S. Pat. No. 6,493,093 propose different methods to correct jumps found in height maps calculated from phase data. Both methods use only one phase profile.